A **quadratic refinement** of a modular curve $X_H$ is a modular cover $X_{H'} \to X_H$ such that $H'<H$ and $H=\{\pm 1\}H'$, so $(H:H')=2$. We also refer to $H'$ as a **quadratic refinement** of $H$. In particular, if $-1 \not \in H$ then there are no quadratic refinements.

A quadratic refinement $X_{H'} \to X_H$ is an isomorphism of the underlying modular curves, but refines the moduli problem from $H$ to $H'$.

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- Review status: beta
- Last edited by John Voight on 2022-11-06 14:35:01

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- 2022-11-06 14:35:01 by John Voight
- 2022-11-06 14:26:29 by John Voight
- 2022-11-06 14:25:02 by John Voight
- 2022-11-06 14:17:00 by John Voight

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